Spatial interpolation of daily potential evapotranspiration for New Zealand using a spline model

Potential evapotranspiration (PET) is an important component of water balance calculations, and these calculations form an equally important role in applications such as irrigation scheduling, pasture productivity forecasts, and groundwater recharge and streamflow modeling. This paper describes a method of interpolating daily PET data calculated at climate stations throughout New Zealand onto a regular 0.05 degrees latitude-longitude grid using a thin-plate smoothing spline model. Maximum use is made of observational data by combining both Penman and Priestley-Taylor PET calculations and raised pan evaporation measurements. An analysis of the interpolation error using 20 validation sites shows that the average root-mean-square error varies between about 1 mm in the summer months to about 0.4 mm in winter. It is advised that interpolated data for areas above 500-m elevation should be used with caution, however, due to the paucity of input data from high-elevation sites.